1. Field of the Invention
The present invention relates to an intake muffler.
2. Description of Related Art
An intake muffler is provided to, for example, an intake air pipe (an air conductive member) of an internal combustion engine to reduce a level of a noisy sound at the intake air pipe. The intake air pipe conducts the noisy sound at multiple frequencies, which change in response to, for example, a rotational speed of the engine. In order to reduce the level of the noisy sound at the intake air pipe, a resonator is provided to the intake muffler. The resonator reduces the level of the sound at a specific frequency through use of the resonance theory of Helmholtz. As shown in FIG. 23, one previously proposed resonator 84 has a resonant chamber 83, which is communicated with an air passage 81 of an intake air pipe 80 through a communication passage 82. The resonant chamber 83 can limit a sound at a corresponding frequency, which is expressed by an equation of K×(S/(L×V))1/2. Here, “K” denotes a constant, and “L” denotes a length of the communication passage 82. Furthermore, “S” denotes a cross sectional area of the communication passage 82, and “V” denotes a volume of the resonant chamber 83. When “S”, “L” and “V” of the above equation are specific characteristic values, the frequency is limited to a specific value. Thus, in order to reduce the level of the noisy sound at the multiple frequencies, multiple resonators need to be provided to the intake air pipe. In general, two or three resonators are provided to the intake air pipe. However, a space of an engine room of a vehicle is limited, and thereby it is often difficult to provides the multiple resonators in the engine room. Also, each of the resonators needs to be placed at the corresponding position, which corresponds to the amplitude of the subject frequency of the sound in the intake air pipe. Thus, the number of counteractable frequencies of the noisy sound is narrowly limited.
Beside the use of the multiple resonators, another technique for reducing the level of the sound is known. According to this technique, a counteracting sound, which has the same frequency as the subject frequency of the noisy sound but has an opposite phase, is generated by forcefully vibrating a diaphragm. When the diaphragm is considered as a spring mass vibration system, a mass of a vibrating part of the diaphragm is denoted by “m”, and an equivalent spring constant of the diaphragm, which is now considered as the spring, is denoted by “k”. An eigenfrequency of the diaphragm can be expressed by (k/m)1/2. Based on this, it is understandable that the equivalent spring constant “k” and/or the mass “m” of the vibrating part of the diaphragm may be changed to change the eigenfrequency of the diaphragm and thereby to counteract with the multiple frequencies. For example, Japanese Unexamined Patent Publication No. 2004-293365 discloses an apparatus that includes an actuator, which changes an eigenfrequency of a diaphragm provided to an intake air pipe. The actuator rotates a depressing bar, which is fixed to or contacts the diaphragm to change a tensile force that is applied to the diaphragm. When the tensile force is changed, the equivalent spring constant k is changed to change the eigenfrequency of the diaphragm. In this way, the multiple frequencies of the noisy sound in the intake air pipe can be attenuated with the single diaphragm and the actuator.
In Japanese Unexamined patent Publication No. 2004-293365, the actuator, which changes the eigenfrequency of the diaphragm, is received in a casing. Furthermore, a motor, the depressing bar and gears for transmitting a rotational force of the motor to the depressing bar are also arranged in the casing. In this instance, the mechanism of converting the rotational force of the motor to the eigenfrequency of the diaphragm is complicated and requires a substantial installation space. In addition, a mechanism of supplying the electric power to drive the motor is required. Thus, when the casing and the mechanism of supplying the electric power to the casing are installed in the engine room of the vehicle, the engine room is further crowded, and manufacturing costs may be increased.
In another case recited in Japanese Unexamined Patent Publication No. H09-264213, air is contained in a resonant chamber of a resonator, which is provided adjacent to a surge tank in an intake air passage that supplies intake air to an internal combustion engine. In the case where the surge tank and the resonator are placed adjacent to each other, when backfire is generated in the engine, a flame, which is generated by the backfire, may possibly be conducted into the resonant chamber through the intake air passage. When this happens, the pressure of the resonant chamber, which forms a closed space, is increased to damage the resonator. In order to limit the damage of the resonator by improving pressure resistivity of the resonator, it is considerable to increase a strength of a connection between the surge tank and the resonator or to increase a wall thickness of the resonator, which forms the resonant chamber. However, in such a case, the increase in the wall thickness of the resonator may disadvantageously cause an increase in the size of the resonator.